A degradation model for blue fluorescent OLEDs for compensation of decreased pixel luminance and lifetime enhancement of OLED displays
Image and Language Processing, Computer Graphics and Visualisation, Human Computer Interaction, Ubiquitous and Wearable Computing
Final Report Abstract
The basis for increasing an OLED’s lifetime is using an accurate degradation model, which determines the present efficiency by electronic means and enables compensation of aged OLED pixels. The latest OLED devices provided by Merck and Novaled were examined for I-V-L (luminance) characteristics during lifetime tests. The blue fluorescent OLEDs and green phosphorescent OLEDs, showed similar aging behavior. It is worth mentioning that the decrease of efficiency in different aging stages strongly depends on the operating point. This fact is highly relevant for aging compensation and at the same time poses a serious challenge for the degradation model. In this project, a correlation model and a data counting model were developed. The correlation model connects the relative change in current efficiency with a relative change in OLED current. The (R2) correlation is very high when the operating point is in the middle or lower operation range. At the beginning of the degradation process, the deviation is large, thus an offset must be inserted into the linear correlation. The state-of-the-art data counting model has been expanded so that the current amplitude and the temperature are considered in the stress induced by operation. The new data counting model with this stress function can reproduce the aging behavior under different operating conditions. The next step is to combine these two models. In the initial operating phase, the data counting model has a high weight in the combination model, while the correlation model gradually gains weight with further operation time. The respective strengths of the two models can thus be deployed. However, the combination model only applies to a defined operating point. A completely new electro-optical model was set up, which essentially consists of two parallel diodes, namely the emitting and the non-emitting diode for the emission layer. The current distribution between the two diodes depends on the operating point. Since the luminance is proportional to the current of the emitting diode, the efficiency depends on the operating point. The aging behavior of the OLED can be described by drifting parameters. The saturation current of the emitting diode is functionally linked to the efficiency from the combination model. With extensive offline simulations, the relative efficiency can be determined by using a 2D lookup table as a function of the aging states and operating point. Thus, we have a degradation model which allows the compensation of aged pixels. The original gray value may be enhanced to reach the target luminance. The compensation for analog driving, which is the standard method for controlling AMOLED displays, poses a particular problem. A gray value altered also means a changed operating point. Based on an implicit equation for the target luminance, binary search method is applied to attain a solution within a limited number of calculation steps. Since the degradation model is dependent on the operating point and is valid over several orders of magnitude, gray values in a wide range can be compensated. The degradation model has been validated with OLED devices. The compensation algorithm was implemented on an FPGA which may control a small AMOLED display. The image sticking artifact on displays can be effectively suppressed, thereby increasing the lifetime of a display significantly (> 10 times). The work has received serious interest from the community and industry. The model and the method investigated in this project can also be applied to the emerging display technology, micro-LEDs.
Publications
- Degradation behavior of blue OLEDs. SID Symp Dig Tech Pap. 2015; 46(S1):62–62
Jiang X, Volkert P, Xu C
(See online at https://doi.org/10.1002/sdtp.10538) - “Characterization and compensation of OLED aging in a digital AMOLED system”, Journal of SID, December 2015, Volume 23, Issue 12, p 570–579
P. Volkert, X. Jiang, C. Xu
(See online at https://doi.org/10.1002/jsid.401) - "Data-Counting Model for Empirical Prediction of OLED Degradation", IDW/AD16, Japan, December 2016, Conference Record, p 651-653
Xingtong Jiang, Chihao Xu
- “Active matrix organic light-emitting diode display and method for driving the same”, US11024214B2, 2016
Chihao Xu, Pascal Volkert
- "Acquiring longer AMOLED Lifetime with Digital Aging Compensation", SID Symposium, USA, May 2017, Digest of Technical Papers, p 207-210
Chihao Xu, Pascal Volkert, Xingtong Jiang
(See online at https://doi.org/10.1002/sdtp.11658) - "An Electro-optical OLED Model for Prediction and Compensation of AMOLED Aging Artifacts", SID Symposium, USA, May 2018, Digest of Technical Papers, p 441-444
Xingtong Jiang, Chihao Xu
(See online at https://doi.org/10.1002/sdtp.12595) - Image Processing for Enhancing Display Performance, IMID 2018, Korea, August 2018
Chihao Xu, Maxim Schmidt, Michael Grüning, Xingtong Jiang
(See online at https://doi.org/10.1109/JSEN.2020.3044392) - A unified OLED aging model combining three modeling approaches for extending AMOLED lifetime. J Soc Inf Display. 2021;1–17
Jiang X, Xu C
(See online at https://doi.org/10.1002/jsid.1064)
